Advances in the catalyst arts have greatly improved efficiencies of catalysts used in the fluidized catalytic cracking of hydrocarbons. As a result, much attention has been devoted to the separation of such catalysts from the effluent gas. Recent developments known as "closed" cyclone technology are disclosed in U.S. Pat. No. 4,909,993, to Haddad et al. which is hereby incorporated herein by reference. Briefly, the output of a reactor riser zone is fed to a primary cyclone separator which is connected in series by a conduit to a second cyclone separator. The conduit is said to prevent random post-riser thermal cracking of the hydrocarbons after they exit the primary cyclone separator by passing the gas directly to the second cyclone without additional residence time in the reaction vessel. The conduit contains an annular port to introduce a stripping gas to improve hydrocarbon/catalyst separation. As described in the Haddad reference, gas leaving the secondary cyclone separator can be passed directly to a tertiary cyclone separator by a pipe connection before leaving the reaction vessel in an exit port. Fourth, fifth, etc. stages can be used in series.
Catalyst separated in the primary cyclone drops through a dipleg attached thereto and passes through a dipleg seal to substantially prevent gas flow through the dipleg. The separated catalyst from each subsequent cyclone separator stage in the series moves down similar cyclone diplegs and accumulates in a bed at the bottom of the reaction vessel. In the bed, stripping steam is used to remove residual hydrocarbons. The stripped catalyst can be removed for conventional reuse or regeneration.
Since most existing hydrocarbon cracking plants were designed using the less efficient catalysts of the past, use of improved catalysts frequently requires retrofitting of an existing catalyst removal system with a design such as that disclosed in the Haddad reference. However, because of limited space in the disengagement vessel and the existing cyclone placement, it is not always possible to achieve efficient catalyst recovery in the retrofit. Often, the existing secondary cyclones must be replaced with expensive, resized newer cyclones.